How to Implement a Multi-Key Dictionary in C#?

Multi-key dictionary in C#

C# itself does not support the data structure of multi-key dictionary. However, some open source libraries provide this functionality.

Tuple-based implementation

A common approach is to use tuples as keys. A tuple is a data structure that represents a collection of multiple values, each with its own type. For example, the following code defines a tuple containing two keys (a string and an integer):

var tuple = (key1: "key1", key2: 1);

You can then use the tuple as the key of the dictionary:

var dictionary = new Dictionary<Tuple<string, int>, string>();
dictionary.Add(tuple, "value");

Structure-based implementation

Another option is to define a custom structure to represent the multi-key value. The following code defines a structure with two fields:

public struct Key
{
    public string Key1 { get; set; }
    public int Key2 { get; set; }
}

You can then use the struct as a key to a dictionary:

var dictionary = new Dictionary<Key, string>();
dictionary.Add(new Key { Key1 = "key1", Key2 = 1 }, "value");

Value object implementation

A more sophisticated approach is to define a class to encapsulate multiple values ​​and enforce value semantics. This approach provides a concise and easy-to-maintain way to handle multi-key data. The following code defines a value object with two fields:

public class ValueObject
{
    public string Key1 { get; private set; }
    public int Key2 { get; private set; }

    public ValueObject(string key1, int key2)
    {
        Key1 = key1;
        Key2 = key2;
    }

    public override bool Equals(object obj)
    {
        return obj is ValueObject other &&
               other.Key1 == Key1 &&
               other.Key2 == Key2;
    }

    public override int GetHashCode()
    {
        return HashCode.Combine(Key1, Key2);
    }
}

You can then use the value object as the key of the dictionary:

var dictionary = new Dictionary<ValueObject, string>();
dictionary.Add(new ValueObject("key1", 1), "value");

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